1. Field of the Invention
The present invention relates to a mode-filtering and mode-selecting method in a multi-mode waveguide, and a waveguide amplifier, a semiconductor laser, and a VCSEL (Vertical-Cavity Surface-Emitting Laser) using the method, and more particularly to an optical unit such as an EDF (Erbium-Doped Fiber), a semiconductor laser, and a VCSEL which include a waveguide having a cladding region that has a periodic index structure, thereby obtaining a mode-filtering or mode-selecting function, which in turn allows only a transverse single mode to propagate along the waveguide.
2. Description of the Related Art
Many researchers are vigorously studying manufacturing methods and the optical transmission characteristics of a photonic crystal fiber that is made by forming a cladding having a structure of periodically arranged holes in a silica glass.
It is known that such a photonic crystal fiber has a marvellous effect that is not easy to explain and distinct from the transmission characteristics of a general optical fiber.
Two main theories have been introduced to explain this effect.
The first is based on a photonic bandgap effect. The second is to use a Helmholtz equation, that is, an electromagnetic wave equation of a refractive-index structure, to calculate an effective refractive index thereof.
The first theory based on the photonic bandgap effect reasons that the lattice structure of cladding holes forms a band gap which is a range of not allowing the transmission of light having a propagation vector that satisfies the Bragg condition.
A numerical approach can be used for the second theory based on the effective refractive index, and therefore many researchers have published reports on the second theory.
According to the second theory, the holes formed in the cladding of the optical fiber function to lower the average refractive index of the cladding to less than the refractive index of the silica.
Therefore, the incident light experiences a relatively lower refractive index in the core having no hole, thereby allowing propagation of the incident light.